Rhodizonic acid
Encyclopedia
Rhodizonic acid is a chemical compound
with formula C6H2O6 or (CO)4(COH)2. It can be seen as a two-fold alcohol
and four-fold ketone
of cyclohexene
, more precisely 5,6-dihydroxycyclohex-5-ene-1,2,3,4-tetrone.
Rhodizonic acid is usually obtained in the form of a colorless "dihydrate" C6H2O6·2H2O. The latter is actually 2,3,5,5,6,6-hexahydroxycyclohex-2-ene-1,4-dione, where two of the original ketone groups are replaced by two pairs of geminal hydroxyls. The orange to deep-red and highly hygroscopic anhydrous acid can be obtained by low-pressure sublimation of the dihydrate.
Like many other alcohols, rhodizonic acid can lose the hydrogen cations
H+ from the hydroxyls (pK1 = 4.378 ± 0.009, pK2 = 4.652 ± 0.014 at 25°C), yielding the hydrogenrhodizonate anion C6HO6− and the rhodizonate anion C6O62−. The latter is aromatic and symmetric, as the double bond and the negative charges are delocalized and evenly distributed over the six CO units. Rhodizonates tend to have various shades of red, from yellowish to purplish.
Rhodizonic acid has been used in chemical assays for barium
, lead
, and other metals. In particular, the sodium rhodizonate test can be used to detect gunshot residue
(which contains lead) in a subject's hands, and to distinguish arrow
wounds from gunshot wounds for hunting regulation enforcement.
in 1837, by analyzing the products of heating a mixture of potassium carbonate and charcoal. For a while, rhodizonates were believed to have the formula C7O7 · Me3. The name comes from Greek
ῥοδίζω (rhodizō, "to tinge red"), on account of the color of its salts.
Potassium
rhodizonate can be prepared with good yield and purity by oxidizing inositol
with nitric acid
and reacting the result with potassium acetate
in the presence of oxygen. The rhodizonate crystallizes out of the solution due to its relative insolubility in water.
Sodium
rhodizonate is dark brown and stable when dry, but the aqueous solution decomposes in a few days, even in the refrigerator. Lead
rhodizonate is dark violet.
(COH)6, tetrahydroxybenzoquinone (THBQ) (COH)4(CO)2, rhodizonic acid (COH)2(CO)4, and cyclohexanehexone
(CO)6.. Lithium rhodizonate, together with salts of THBQ and benzenehexol, has been considered for possible use in rechargeable electrical batteries. The monovalent anion has been detected in mass spectrometry experiments.
Rhodizonic acid and the rhodizonate anion can lose one of the CO units to yield croconic acid
(CO)3(COH)2 and the croconate anion C5O52−, respectively, by mechanisms that are still imperfectly known. In basic solutions (pH > 10), rhodizonic acid quickly converts to the THBQ anion (CO)64− in the absence of oxygen, and to croconic acid in its presence. At pH 8.3 and exposure to light, solutions are stable for days in the absence of oxygen, and decompose to croconic acid and other products (possibly including cyclohexanehexone
or dodecahydroxycyclohexane
) in its presence.
rhodizonate 2Rb+ · C6O62−, the rhodizonate anions are stacked in parallel columns, and so are the rubidium ions. In the plane perpendicular to the columns, these are arranged as two interleaved hexagonal grids. The anions are planar.
Anhydrous potassium rhodizonate 2K+ · C6O62− has a distinct but similar structure. The anions and cations are arranged in alternate planes. Within each plane, the anions are arranged in an haexaginal grid. Each K ion is arranged so that it connects symmetrically to eight oxygens of four anions, two from each adjacent plane. The anions are slightly twisted in the "boat" shape (with 0.108 Å of rms deviation from mean plane). Sodium rhodizonate 2Na+ · C6O62− has the same structure, with slightly more distorted anions (0.113 Å rms)
In solution, the rhodizonate anion is not hydrated.
Chemical compound
A chemical compound is a pure chemical substance consisting of two or more different chemical elements that can be separated into simpler substances by chemical reactions. Chemical compounds have a unique and defined chemical structure; they consist of a fixed ratio of atoms that are held together...
with formula C6H2O6 or (CO)4(COH)2. It can be seen as a two-fold alcohol
Alcohol
In chemistry, an alcohol is an organic compound in which the hydroxy functional group is bound to a carbon atom. In particular, this carbon center should be saturated, having single bonds to three other atoms....
and four-fold ketone
Ketone
In organic chemistry, a ketone is an organic compound with the structure RCR', where R and R' can be a variety of atoms and groups of atoms. It features a carbonyl group bonded to two other carbon atoms. Many ketones are known and many are of great importance in industry and in biology...
of cyclohexene
Cyclohexene
Cyclohexene is a hydrocarbon with the formula C6H10. This cycloalkene is a colorless liquid with a sharp smell. It is an intermediate in various industrial processes...
, more precisely 5,6-dihydroxycyclohex-5-ene-1,2,3,4-tetrone.
Rhodizonic acid is usually obtained in the form of a colorless "dihydrate" C6H2O6·2H2O. The latter is actually 2,3,5,5,6,6-hexahydroxycyclohex-2-ene-1,4-dione, where two of the original ketone groups are replaced by two pairs of geminal hydroxyls. The orange to deep-red and highly hygroscopic anhydrous acid can be obtained by low-pressure sublimation of the dihydrate.
Like many other alcohols, rhodizonic acid can lose the hydrogen cations
Proton
The proton is a subatomic particle with the symbol or and a positive electric charge of 1 elementary charge. One or more protons are present in the nucleus of each atom, along with neutrons. The number of protons in each atom is its atomic number....
H+ from the hydroxyls (pK1 = 4.378 ± 0.009, pK2 = 4.652 ± 0.014 at 25°C), yielding the hydrogenrhodizonate anion C6HO6− and the rhodizonate anion C6O62−. The latter is aromatic and symmetric, as the double bond and the negative charges are delocalized and evenly distributed over the six CO units. Rhodizonates tend to have various shades of red, from yellowish to purplish.
Rhodizonic acid has been used in chemical assays for barium
Barium
Barium is a chemical element with the symbol Ba and atomic number 56. It is the fifth element in Group 2, a soft silvery metallic alkaline earth metal. Barium is never found in nature in its pure form due to its reactivity with air. Its oxide is historically known as baryta but it reacts with...
, lead
Lead
Lead is a main-group element in the carbon group with the symbol Pb and atomic number 82. Lead is a soft, malleable poor metal. It is also counted as one of the heavy metals. Metallic lead has a bluish-white color after being freshly cut, but it soon tarnishes to a dull grayish color when exposed...
, and other metals. In particular, the sodium rhodizonate test can be used to detect gunshot residue
Gunshot residue
Gunshot residue is principally composed of burnt and unburnt particles from the explosive primer, the propellant, as well as components from the bullet, the cartridge case and the firearm used...
(which contains lead) in a subject's hands, and to distinguish arrow
Arrow
An arrow is a shafted projectile that is shot with a bow. It predates recorded history and is common to most cultures.An arrow usually consists of a shaft with an arrowhead attached to the front end, with fletchings and a nock at the other.- History:...
wounds from gunshot wounds for hunting regulation enforcement.
History
Rhodizonic acid was discovered by Austrian chemist Johann HellerJohann Florian Heller
Johann Florian Heller was an Austrian chemist who was one of the founders of clinical chemistry.Heller was born in Vienna, Austria. He studied chemistry in Prague and later with Liebig and Wöhler at Giessen...
in 1837, by analyzing the products of heating a mixture of potassium carbonate and charcoal. For a while, rhodizonates were believed to have the formula C7O7 · Me3. The name comes from Greek
Greek language
Greek is an independent branch of the Indo-European family of languages. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. Its writing system has been the Greek alphabet for the majority of its history;...
ῥοδίζω (rhodizō, "to tinge red"), on account of the color of its salts.
Salts
Rhodizonates tend to have various shades of red, from yellowish to purplish, in transmitted light, with a greenish luster in reflected light.Potassium
Potassium
Potassium is the chemical element with the symbol K and atomic number 19. Elemental potassium is a soft silvery-white alkali metal that oxidizes rapidly in air and is very reactive with water, generating sufficient heat to ignite the hydrogen emitted in the reaction.Potassium and sodium are...
rhodizonate can be prepared with good yield and purity by oxidizing inositol
Inositol
Inositol or cyclohexane-1,2,3,4,5,6-hexol is a chemical compound with formula 6126 or 6, a sixfold alcohol of cyclohexane. It exists in nine possible stereoisomers, of which the most prominent form, widely occurring in nature, is cis-1,2,3,5-trans-4,6-cyclohexanehexol, or myo-inositol...
with nitric acid
Nitric acid
Nitric acid , also known as aqua fortis and spirit of nitre, is a highly corrosive and toxic strong acid.Colorless when pure, older samples tend to acquire a yellow cast due to the accumulation of oxides of nitrogen. If the solution contains more than 86% nitric acid, it is referred to as fuming...
and reacting the result with potassium acetate
Potassium acetate
Potassium acetate is the potassium salt of acetic acid.-Preparation:It can be prepared by reacting a potassium-containing base such as potassium hydroxide or potassium carbonate with acetic acid:...
in the presence of oxygen. The rhodizonate crystallizes out of the solution due to its relative insolubility in water.
Sodium
Sodium
Sodium is a chemical element with the symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals; its only stable isotope is 23Na. It is an abundant element that exists in numerous minerals, most commonly as sodium chloride...
rhodizonate is dark brown and stable when dry, but the aqueous solution decomposes in a few days, even in the refrigerator. Lead
Lead
Lead is a main-group element in the carbon group with the symbol Pb and atomic number 82. Lead is a soft, malleable poor metal. It is also counted as one of the heavy metals. Metallic lead has a bluish-white color after being freshly cut, but it soon tarnishes to a dull grayish color when exposed...
rhodizonate is dark violet.
Oxidation and decomposition
Rhodizonic acid is a member of a chain of oxidation products: benzenehexolBenzenehexol
Benzenehexol, also called hexahydroxybenzene, is an organic compound with formula C6H6O6 or C66. It is a six-fold alcohol of benzene. The product is also erroneously called hexaphenol, but this name has been used also for other substances....
(COH)6, tetrahydroxybenzoquinone (THBQ) (COH)4(CO)2, rhodizonic acid (COH)2(CO)4, and cyclohexanehexone
Cyclohexanehexone
Cyclohexanehexone, also known as hexaketocyclohexane and triquinoyl, is a organic compound with formula C6O6, the sixfold ketone of cyclohexane...
(CO)6.. Lithium rhodizonate, together with salts of THBQ and benzenehexol, has been considered for possible use in rechargeable electrical batteries. The monovalent anion has been detected in mass spectrometry experiments.
Rhodizonic acid and the rhodizonate anion can lose one of the CO units to yield croconic acid
Croconic acid
Croconic acid or 4,5-dihydroxycyclopentenetrione is a chemical compound with formula C5H2O5 or 32. It has a cyclopentene backbone with two hydroxyl groups adjacent to the double bond and three ketone groups on the remaining carbon atoms...
(CO)3(COH)2 and the croconate anion C5O52−, respectively, by mechanisms that are still imperfectly known. In basic solutions (pH > 10), rhodizonic acid quickly converts to the THBQ anion (CO)64− in the absence of oxygen, and to croconic acid in its presence. At pH 8.3 and exposure to light, solutions are stable for days in the absence of oxygen, and decompose to croconic acid and other products (possibly including cyclohexanehexone
Cyclohexanehexone
Cyclohexanehexone, also known as hexaketocyclohexane and triquinoyl, is a organic compound with formula C6O6, the sixfold ketone of cyclohexane...
or dodecahydroxycyclohexane
Dodecahydroxycyclohexane
Dodecahydroxycyclohexane is an organic compound with molecular formula C6O12H12 or C612. It is a sixfold geminal diol with a cyclohexane backbone and can be regarded as a sixfold hydrate of cyclohexanehexone .-Dihydrate:...
) in its presence.
Acid
In solution, the acid and the hydrogenrhodizonate ion are mostly hydrated, with some of the ketone groups C=O replaced by geminal hydroxyls, C(OH)2Salts
In anhydrous rubidiumRubidium
Rubidium is a chemical element with the symbol Rb and atomic number 37. Rubidium is a soft, silvery-white metallic element of the alkali metal group. Its atomic mass is 85.4678. Elemental rubidium is highly reactive, with properties similar to those of other elements in group 1, such as very rapid...
rhodizonate 2Rb+ · C6O62−, the rhodizonate anions are stacked in parallel columns, and so are the rubidium ions. In the plane perpendicular to the columns, these are arranged as two interleaved hexagonal grids. The anions are planar.
Anhydrous potassium rhodizonate 2K+ · C6O62− has a distinct but similar structure. The anions and cations are arranged in alternate planes. Within each plane, the anions are arranged in an haexaginal grid. Each K ion is arranged so that it connects symmetrically to eight oxygens of four anions, two from each adjacent plane. The anions are slightly twisted in the "boat" shape (with 0.108 Å of rms deviation from mean plane). Sodium rhodizonate 2Na+ · C6O62− has the same structure, with slightly more distorted anions (0.113 Å rms)
In solution, the rhodizonate anion is not hydrated.
See also
- AcetylenediolAcetylenediolAcetylenediol, or ethynediol, is a chemical substance with formula HO-C≡C-OH. It is the diol of acetylene. Acetylenediol is unstable in the condensed phase, although its tautomer glyoxal H2H is well known.-Detection:...
(COH)2 - Deltic acidDeltic acidDeltic acid or dihydroxycyclopropenone is a chemical substance with the chemical formula C3O2. It can be viewed as a ketone and double alcohol of cyclopropene...
(CO)(COH)2 - Squaric acidSquaric acidSquaric acid, also called quadratic acid, because its four carbon atoms approximately form a square, is an organic compound with chemical formula 424....
(CO)2(COH)2 - Croconic acidCroconic acidCroconic acid or 4,5-dihydroxycyclopentenetrione is a chemical compound with formula C5H2O5 or 32. It has a cyclopentene backbone with two hydroxyl groups adjacent to the double bond and three ketone groups on the remaining carbon atoms...
(CO)3(COH)2